There are varieties of electro-optical (E/O) imaging systems used in commercial and military fields for reconnaissance and surveillance. For optimal status awareness and target discrimination, images acquired using various imaging detectors with sufficient target details and wide field of view (FOV) are necessary.
Similar to the human eye, an E/O imaging system also consists of optical imaging lens, photo sensing array and image receiving and processing unit. Optical imaging lenses of both human eye and E/O imaging system could have very powerful resolution capability; however, the later has much poorer overall performance. The basic reason is that the human eye has foveated imaging feature, which has a high resolution central foveal region. Surrounding this region, gradually reduced resolution extends to a wide FOV for each eye [Ref 1: C. Curcio, K. Sloan, O packer, A. Hendrickson, R. Kalina, “Distribution of cones in human and monkey retina: individual variability and radial asymmetry.” Science 236, 579-582 (1987)]. The eye gazes at the region of interest (ROI) and images ROI on foveal region to achieve highest resolution while remains awareness at surroundings with lower resolution. Furthermore, the eyeball is capable of rotating rapidly about its center to reposition its fovea. The brain then integrates over time the information entering the small but very agile high resolution-density foveal region to create the subjective impression of a uniform-density wide FOV high resolution image.
Contrastively, a conventional E/O imaging system has uniform density pixel on entire sensing region. To image a wide FOV scene, it would be necessary to combine vast amount of individual images into a giant mosaic due to the limited format size of a conventional imaging detector device. There is a trade-off between the achievable viewing angle and the resolving ability, i.e. wide FOV results in coarse resolution and vice versa.
Practical applications require both wide FOV and high resolution of an E/O imaging sensor. For example, surveillance and tracking in urban warfare or in civilian commercial security, two basic functions are necessary. The first is the detection of the potential existence of a specific object within a wide area. The second function is to identify and track or point the object with high accuracy. In the event of multiple objects, the number of targets needs to be counted and tracked at substantially the same time.
Based-on the available pixel limited detector array, some wide FOV and high resolution imaging sensors have been developed. It has been reported that target tracking system uses two independent optical imaging systems to respectively acquire global coarse scene and local fine interest area [Ref: 2: J. M. Denney, E. L. Upton, “Missile surveillance method and apparatus,” U.S. Pat. No. 5,300,780]. Once a target is detected in the coarse image, a control signal is sent to the fine optical imaging system to set it pointing to the interested direction. Its advantage is that using two detector arrays of moderate resolution can acquire scene in interested area with high resolution. The shortcomings are that the adjustment of the fine imaging system is mechanical, which is subject to problems of accuracy, vibration, calibration, drift, and unwanted resonance. If there are more than one targets, either several fine imaging systems are needed or using one fine imaging system to perform sequential checking each interested local area. This results in a complex system with slow response.
With the increase in transistor density in CMOS arrays, the future sensor arrays may have much more pixel number to 100+ megapixels per sensor for acquiring high-resolution images with wide FOV. However, when these imaging arrays grow in pixel number for higher resolution, very strong onboard processing and high communication bandwidths will also be required to acquire the image and/or transmit the image data between platforms. There will be a limitation for applications in small payload platforms that require low cost, compact packaging and low power consumption.
Based on above discussions, there are mainly two constraints in realization of high resolution, large coverage and fast frame rate imaging sensor, which are sensor pixel numbers and data link bandwidth. This situation implies a demand to develop innovative sensor architecture, based on available imaging sensor format size, that can achieve simultaneously wide FOV monitoring with low resolution and local high resolution for object discrimination.